WG C4.46 Evaluation of Temporary Overvoltages in Power Systems due to Low Order Harmonic Resonances


The introduction of long HVAC underground cables and the integration of large offshore wind park projects at transmission levels via HVAC export cables impose important challenges to the System Operators. One of these challenges is related to the low harmonic order resonance frequencies. Several detailed electromagnetic transient (EMT) studies have shown that once excited, these resonance frequencies -especially the ones close to the 2nd harmonic-  might result in Temporary Overvoltages (TOVs), which are characterized by longer durations and elevated amplitudes.

Although this type of overvoltages could impose severe dielectric and thermal stresses on equipment, they are not covered by International standards. With the exception of the metal-oxide surge arresters, there are also no standardized testing procedures related to the TOV capability of the equipment. Consequently, the withstand characteristics of already installed equipment is also unclear, as these were not included in the specifications. In the past, CIGRE WG33.10 published generic TOV withstand characteristics of High Voltage apparatus. These characteristics consider voltages of the fundamental power frequency, excluding the impact of harmonic distortion. Therefore, it is important that guidelines/methods to evaluate the TOVs in transmission systems are developed. These methods and their outcome could have a great impact on the decision-making during the planning process, by indicating whether remedial measures need to be considered against excessive TOVs.


The Working Group will establish methods and guidelines regarding the evaluation of TOVs in transmission systems. The main tasks of the WG are as follows: 

  1. Review of existing standards and guidelines related to the impact of harmonic distortion on TOVs.
  2. Review of existing standards and testing procedures of High Voltage equipment related to TOVs.
  3. Review practices of TOV evaluation currently applied by utilities.
  4. Specify criteria related to the evaluation of TOVs. The proposed criteria will be related to the dielectric and thermal impact of TOVs on the HV equipment.
  5. Specify method(s) for analysing TOVs, as calculated through detailed system simulation models, and evaluating them based on the specified criteria.


  Technical Brochure and Executive summary in Electra

  Electra report


Time Schedule: start: September 2017 

Final Report: December 2019

Convner: Filipe Faria da Silva  (Denmark)